ctdb-ltdb: Use tdb_null instead of zeroing TDB_DATA variable

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Volker Lendecke <vl@samba.org>

Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Jul 14 16:01:31 CEST 2014 on sn-devel-104
As David Woodhouse points out, this breaks backwards compatibility.

https://bugzilla.samba.org/show_bug.cgi?id=10692

Revert "libwbclient: reject unknown named blobs in wbcCredentialCache()"

This reverts commit 740d12d1e77d356ff22c3725dce8d5019c86a7a5.

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Mon Jul 14 21:54:08 CEST 2014 on sn-devel-104